In South Africa, an estimated 18% of the entire population age 15-49 is HIV positive, including approximately 10% of young women aged 15-24;this epidemic is driven almost entirely by heterosexual transmission. One key dynamic affecting heterosexual HIV transmission in South Africa, particularly among young people, is financial and material exchange as an underlying motive for sexual relationships. Often referred to as transactional sex, such exchange can be an incentive for women to have sex in situations where they might otherwise refrain, and has been noted as a potential source of women's vulnerability to gender- based violence and sexual exploitation. Research with young South African men has highlighted associations between transactional sex and perpetration of both sexual assault and intimate partner violence. Transactional sex is also associated with a range of other behaviors, such as multiple and concurrent sexual partnerships and sex while intoxicated, that carry potential for increased risk of HIV infection. In South Africa, transactional sex and gender-based violence have both been associated with prevalent HIV infection in cross-sectional studies, underscoring the importance to HIV prevention of better understanding these linked phenomena. Because research on transactional sex, gender-based violence, and HIV risk has to date been only cross-sectional, our understanding of possible causal associations and potential points of intervention remains severely limited. The recently completed Stepping Stones HIV prevention trial (5R01MH064882-04;3R01MH064882-04S2) offers a unique opportunity to explore associations between transactional sex, gender-based violence, and incident HIV and HSV-2 infection through secondary analysis of longitudinal data from a large HIV prevention trial with youth in the rural Eastern Cape Province of South Africa. The specific aims of this project are to: (1) Assess whether experience (for women) or perpetration (for men) of gender-based violence predicts participation in transactional sex over 24 months of follow-up among 1,252 young, rural South African women and 1,179 young, rural South African men who participated in the Stepping Stones trial and for whom follow-up data is available;(2) Assess whether participation in transactional sex predicts experience/perpetration of gender- based violence among rural South African youth over 24 months of follow-up;(3) Assess whether participation in transactional sex predicts incident HIV or HSV-2 infection over 24 months of follow-up among 1,126 young rural South African women and 1,130 young rural South African men who participated in the Stepping Stones study and have viral incidence data available;and (4) Assess whether experience or perpetration of gender- based violence predicts incident HIV or HSV-2 infection over 24 months of follow-up among rural South African youth. This study will answer key questions about the role of transactional sex and gender-based violence in shaping young people's risk of HIV infection and inform future interventions to reduce the risk of both HIV and gender-based violence among African youth. PUBLIC HEALTH RELEVANCE: South Africa has the highest number of HIV infected individuals in the world, with an epidemic driven by heterosexual transmission and reinforced by high incidence of gender-based violence and inequality. Young people who are just becoming sexually active represent both a pool of extreme vulnerability and a potentially valuable point for interventions for prevent both HIV and gender-based violence. This analyses proposed here will yield a deeper understanding of the linked role of transactional sex and gender-based violence among South African youth and the extent to which transactional sex and gender- based violence impact women's and men's risk of acquiring HIV and/or HSV-2 infection, and will thereby inform development of improved prevention interventions.